Asymmetrical driver

ABSTRACT

A drive circuit having asymmetrical drivers. In an embodiment, a brushless DC motor may be driven by a drive circuit having three high-side MOSFETs and three low-side MOSFETs. A driver controller turns the MOSFETs on and off according to a drive algorithm such that phase currents are injected into motor coils to be driven. The high-side MOSFETs may be sized differently than the low-side MOSFETs. As such, when a MacDonald waveform (or similar drive algorithm) is used to drive the phases of the motor, less power may be required during disk spin-up because the MOSFETs that are on more (e.g., the low-side MOSFETs with a MacDonald waveform) may be sized larger than the MOSFETs that are on less (e.g., the high-side MOSFETs). In this manner, less power is dissipated in the larger size MOSFETs that are on more than the others.

PRIORITY CLAIM TO PROVISIONAL PATENT APPLICATION

This patent application claims priority to U.S. Provisional PatentApplication No. 61/115,870 entitled ‘ASYMMETRICAL DRIVER,’ and which wasfiled on Nov. 18, 2008 and is hereby incorporated by reference.

BACKGROUND

Hard disk drives (HDD) are becoming smaller while at the same timeproviding greater storage capacity. One reason for these advances is themore prevalent use of brushless direct current motors (BLDC motor) torotate the HDD. Further yet, a BLDC motor may be a three-phase motor andbe driven by pulse-width modulation (PWM). For example, U.S. Pat. No.6,137,253, which is incorporated by reference, discloses driving each ofthree BLDC motor coils with a respective PWM signal that causes asinusoidal (or approximately sinusoidal) current to flow through each ofthe coils. By causing phase shifted (by approximately 120°) sinusoidalcurrents to flow through the coils, the BLDC motor is driven with aconstant or approximately constant torque. This may be desirable in anapplication, such as disk drive applications, where it may be desirableto reduce or eliminate torque ripple in the rotation of the motor andthat which the motor is rotating (e.g., a disk).

In one application, BLDC motor coils are driven with what is called aMacDonald voltage wave form, which is a PWM waveform that is describedin U.S. Pat. No. 6,137,253. The MacDonald voltage wave form, when usedto drive the motor coils, causes sinusoidal currents to flow through thecoils. This may be accomplished by using a drive circuit having twodrivers that comprise a high-side driver and a low-side driver. Thus,the Macdonald waveform may be conditioned to hold, for each 120° portionof the electrical period, one of the high-side or low-side drivers(MOSFET transistors in one example) for one of the coils in an ON state.Holding the high-side or low-side driver in an ON state maysignificantly reduce the switching losses in each drive transistor, andthus may significantly reduce the power dissipated by the chip.

While it may be desired that the spindle differential phase-to-phasecurrent waveform be as symmetrical as possible (sinusoid) in order tohave a constant spindle torque, there is no requirement that theabsolute spindle phase voltage be held to a specific voltage to achievethis. Therefore, driver designers use this freedom to improve the driveroperation through various drive algorithms that drive each phase pair ordrivers, which may lead to reducing the switching losses, for example.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the subject matter disclosed herein will become morereadily appreciated as the same become better understood by reference tothe following detailed description, when taken in conjunction with theaccompanying drawings.

FIG. 1 is a block diagram of an embodiment of a HDD having a motor fordriving a disk with an asymmetrical driver circuit.

FIG. 2 is a schematic diagram of an embodiment of a drive circuit ofFIG. 1 having asymmetrical drivers.

FIG. 3 is a plot of the ratio of asymmetry in the drive transistors ofFIG. 2 against spin-up losses in a hard drive system illustrating anembodiment of a driving method wherein the drive circuit isasymmetrical.

FIG. 4 is a plot of power dissipated in drive transistors of FIG. 2illustrating an embodiment of a driving method wherein the drive circuitis symmetrical.

FIG. 5 is a plot of power dissipated in drive transistors of FIG. 2illustrating an embodiment of a driving method wherein the drive circuitis asymmetrical.

FIG. 6 is a block diagram of an embodiment of computer system having theHDD system of FIG. 1.

DETAILED DESCRIPTION

The following discussion is presented to enable a person skilled in theart to make and use the subject matter disclosed herein. The generalprinciples described herein may be applied to embodiments andapplications other than those detailed above without departing from thespirit and scope of the present detailed description. The presentdisclosure is not intended to be limited to the embodiments shown, butis to be accorded the widest scope consistent with the principles andfeatures disclosed or suggested herein.

FIG. 1 is a block diagram of an embodiment of a HDD having a motor forrotating a disk with an asymmetrical driver circuit. The spindle motor105 may be a three-phase brushless direct-current motor (BLDC motor)that may be driven synchronously by a controller 108, which may controla power chipset 107. Thus, instead of a mechanical commutation systembased on brushes typically found in a brushed DC motor, the brushless DCmotor 105 is commutated using electronic circuitry. Such circuitry mayinclude driver circuits 107 a-107 c for each phase of the BLDC motor105, (e.g., MOSFET transistors as described further below with respectto FIG. 2), which are part of the power chipset 107. Further, the powerchip set 107 may be an integrated circuit disposed on a single die, ormay be multiple integrated circuits and/or non-integrated circuitcomponents disposed on separate dies or otherwise separately from eachother. Other variations of one or more integrated circuit dies arepossible as well. For example, the drive controller 108, the power chipset 107, the read/write channel 117 and other components may all bedisposed on the same integrated circuit die, each component separatelydisposed on separate integrated circuit dies, or any combinationthereof.

The speed of the BLDC motor 105 depends on the voltages applied at itsphases. By varying the average voltage across the phases, therevolutions per minute (RPM) of the BLDC motor 105 may be altered. Thisis achieved by altering the duty cycle of each phase's driver circuit107 a-107 c. Thus, each phase 160 a-c may receive coordinatedpulse-width modulated (PWM) signals having a duty cycle at the drivenodes of the respective driver circuits 107 a-107 c (i.e., gates oftheir respective MOSFETs as shown below in FIG. 2) that are suited toproduce a specific motor speed. Typically, the higher the duty cycle,the higher the speed. Thus, the driver circuits 107 a-107 c may beswitched on and off at a specific rate to produce a desired duty cycle,which, in turn, drives the BLDC motor 105 at a desired speed.

In operation, a power supply 170 provides a voltage to a voltage bus Vm.This voltage may be, for example, 12V for HDD systems in desktopcomputers and may be 5V is HDD systems in laptop computers. This voltagenot only provides power to drive the BLDC motor 105, but also providespower to the other portions of the HDD 100. Other portions of the HDD100 may include read/write channel circuitry 117, read/write headactuation devices 116, the hard disk(s) 115 and the like.

Although described as a HDD system 100 herein, the methods andapplications discussed herein may be applicable to any device having amotor for rotating a disk or other device. For example, a DVD drive, aCD drive, and other similar devices may also employ the methods andtechniques discussed herein. As illustrated below, power savings may berealized by tailoring the specific sizes of the respective components ofeach driver circuit 107 a-107 c.

FIG. 2 is a schematic diagram of an embodiment of a drive circuit ofFIG. 1 having asymmetrical drivers. The drive circuit 107 of FIG. 1 isshown in greater detail wherein each phase's driver circuit 107 a-107 cincludes a respective high-side driver 201, 202, and 203 and arespective low-side driver 211, 212, and 213. Thus, for phase A, thedriver circuit 107 a includes a high-side MOSFET transistor 201 and alow-side MOSFET transistor 211. Similarly, a phase B driver circuit 107b includes a high-side MOSFET transistor 202 and a low-side MOSFETtransistor 212. Finally, a phase C driver circuit 107 c includes ahigh-side MOSFET transistor 203 and a low-side MOSFET transistor 213.The gates of each of these transistors may be coupled to a drivecontroller (108 of FIG. 1) such that each transistor may be turned ON orOFF according to a particular drive waveform (e.g., a MacDonaldwaveform).

These MOSFET transistors compose three sets of bi-directional drivers201/211, 202/212 and 203/213 (these MOSFET transistor pairs may also becalled half bridges) that drive the phases 160 a-c of the BLDC motor105. The drive controller 108 may implement control of the MOSFETtransistors by monitoring a current through sense resistor 230 or asense transistor in parallel with an isolation transistor 220 (discussedbelow), and/or monitoring the supply voltage Vm, and monitoring the BLDCmotor 105 position using conventional sensorless techniques to producedesired phase voltages, and, therefore, a corresponding motor speed.More advanced controllers 108 may employ a microprocessor to manage thedisk's acceleration/deceleration, control the disk's speed, andfine-tune efficiency of delivery of power to actuating the disk.

In many applications, the PWM pulses that represent the MacDonald waveform are asymmetrical (from an absolute reference). That is, the pulsesthat compose the MacDonald voltage may activate one of the high-side andlow-side drive transistors of a phase for less than half of the time,and activate the other of the high-side and low-side drive transistorsfor more than half of the time. The time during which the one drivetransistor is ON verse the time that the other drive transistor is ONdepends, e.g., on the supply voltage Vm that is being switched to thecoil, the rotational speed of the BLDC motor, and the coil currentneeded to maintain that rotational speed.

Thus, in one example embodiment, if the supply voltage Vm is relativelyhigh, then, for each phase, the high-side drive transistor is ON for ashorter duration of time as compared to the amount of time that thelow-side transistor is ON. But for the same disk rotational speed, asthe supply voltage Vm decreases, then the high-side drive transistor maybe ON for a longer duration of time than it is with a higher Vm. Forexample, the high-side transistor may be ON for approximately 48% orless of the drive-transistor ON time during the phase drive cycle, andthe low-side transistor may be ON for approximately 52% or more of thedrive-transistor ON time during the phase drive cycle.

The drive circuit 107 may further include an isolator 220, sometimescalled an isolation transistor or isolation field-effect transistor(isofet), that is disposed between the Vm supply-voltage node and thehigh-side driver transistors 201, 202, and 203. The isofet 220 allowsthe drive controller 108 (FIG. 1) to isolate the supply voltage Vm fromthe high-side transistors 201, 202, and 203 by turning the isofet 220OFF, for example, in case of a short circuit.

Disk-drive manufacturers may specify a value of diagonal RDSon for thedrive circuit 107, where diagonal RDSon is the maximum sum, or a maximumweighted sum of the ON resistances of the isofet 220, a high-sidetransistor, a low-side transistor. In a bridge configuration where twocoils are being driven in series, a current flows from the supplyvoltage node Vm, through the isofet 220, through a high-side transistor,through the coils of a first respective phase and subsequent secondrespective phase, and through a low-side transistor of another H-bridge,to the low-power rail 225. Thus, diagonal RDSon is the maximum combinedseries ON resistance of these two driving transistors and the isofet220. For example, a specification of 0.3 ohms for the diagonal RDSonindicates that when the isofet 220, a first high-side transistor of oneH-bridge (MOSFET 201 for example), and then one diagonally oppositelow-side transistor of another different H-bridge (MOSFET 213, forexample, to cause a current to flow through coils A and C) are all ON,the total path resistance due to these three transistors can be nogreater than 0.3 ohms.

However, there may be no requirement or reason that these transistorsneed to have equivalent RDSon values. That is, with the specification ofa diagonal RDSon of 0.3 ohms, one need not size each transistor (isofet220, high-side MOSFET and low-side MOSFET) to have an RDSon of 0.1 ohms.As such, these transistors may be sized differently. Furthermore, when amotor-controller manufacture specifies a diagonal RDSon for the drivercircuit 107, it typically calculates the diagonal RDSon by addingtogether the RDSon of the isofet, and the duty-cycle weighted RDSons ofany one of the high-side and any one of the low-side transistors for aspecified load current range, because all of the high-side transistorsmay have substantially the same weighted RDSon, as may all of thelow-side transistors. But by sizing the high-side MOSFETs and low-sideMOSFETs with a specific asymmetric ratio, a power savings may berealized because of the asymmetry of the drive algorithm. The specificratio chosen may yield different power savings as shown below in FIG. 3.

FIG. 3 is a plot of the ratio of asymmetry in the drive transistors ofFIG. 2 against the average spin-up losses in an embodiment of an HDDsystem where the drive transistors are pulse-width modulated ON and OFFin an asymmetric fashion according to a McDonald waveform, which causes,on average, the low-side transistors to be on longer than the high-sidetransistors. In this plot, the y-axis represents the power loss duringthe start-up, or spin-up period of the disk being rotated by the motor,and the x-axis represents the factor x of asymmetry between the twohigh- and low-side MOSFETs that, together with the isofet 220, composethe diagonal RDSon as discussed above.

In this embodiment, the low-side transistor has a larger size (e.g.,width), and thus a smaller RDSon, than the high-side transistor, becauseas stated above, the McDonald waveform may dictate that the low-sidetransistor is ON for a longer portion of a phase cycle than thehigh-side transistor. For example, the relative size of the high-sidetransistor is given by 1−x, and the relative size of the low-sidetransistor is given by 1+x. So, where x=0.2, then the relative size ofthe high-side transistor is 1−0.2=0.8, the relative size of the low-sidetransistor is 1+0.2=1.2, such that the low-side transistor is 50% largerthan the high-side transistor. It follows, therefore, that the RDSon ofthe high-side transistor is increased to 1.25 times its value for x=0,and that the RDSon of the low-side transistor is decreased to 0.83 timesits value for x=0, such that the RDSon of the high-side transistor isapproximately 50% larger than the RDSon of the low-side transistor. Inan embodiment, the range of x in FIG. 3 is from x=0, meaning that bothtransistors are sized the same, to x=0.5. As stated above, the y-axisrepresents the total power loss during disk spin-up. Further, one mayassume that each transistor's RDSon value is inversely proportional toits channel width, and proportional to its channel length. In anembodiment where only the width is varied, one may use “width” and“size” interchangeably.

Thus, in the embodiment represented by the plot in FIG. 3, one may seethat that the value of x that yields the lowest average power loss overthe entire spin-up period is approximately x=0.2. Therefore, withreference back to FIGS. 2-3, in an embodiment, one may realize thesmallest losses over the duration of disk spin-up (and over the durationof steady-state disk operation under a similar theory as discussedbelow) by sizing one or more of the H-bridge transistors (the low-sideMOSFETs, for example) to have a smaller RDSon than one or more oppositeH-bridge transistor (the high-side MOSFETs, for example).

During steady-state operation where the motor is operating at asteady-state speed, one may take into account the duty cycle whencalculating the diagonal RDSon.

As a first example, both high and low H-bridge transistors may have thesame RDSon values, e.g., RDSon=0.1 ohms for both the high-side 201 andlow-side 212 transistors. This may be because each transistor is sizedthe same, i.e., x=0. Then, for any duty cycle D, the effective diagonalRDSon of the bridge (excluding the isofet for purposes of examplebecause the isofet experiences a 100% duty cycle) is going to be 0.1ohms because the high-side 201 and low-side 212 transistors aregenerally not ON or OFF at the same time (except perhaps duringnegligible times during zero-crossings of the driving waveform to sensethe motor position). For example, where D=40%, meaning that thehigh-side transistor is ON for approximately D=40% of the cycle and thelow-side transistor is ON for approximately 100%−D=60% of the cycle, theeffective diagonal RDSon=0.4×0.1 (the effective RDSon of the high-sidetransistor)+0.6×0.1 (the effective RDSon of the low-side transistor)=0.1ohms. Therefore, the power-loss through such a drive circuit issubstantially independent on duty cycle.

But where the high- and low-side transistors are asymmetrically sized, aduty-cycle-dependent power savings may be realized. Thus, if thehigh-side 201 and low-side 212 transistors are driven with equal dutycycles, then the effective diagonal RDSon of the bridge is going to bethe sum of the effective RDSons of the high-side and low-sidetransistors divided by two. However, if the duty cycle is shifted tofavor one side over the other, the effective RDSon may yield a relativepower savings as discussed below.

To illustrate this further, assume a low-side transistor 212 has anRDSon of 0.1 ohms and a high-side transistor 201 has an RDSon of 0.2ohms. At a 50% duty cycle, the effective diagonal RDSon of these twotransistors is 0.15 ohms per above. But now assume a 40% duty cycle,wherein the low-side transistor 212 is ON approximately 60% of the cycletime and the high-side transistor 201 is ON approximately 40% of thecycle time. This results in an effective diagonal RDSon of 0.6×0.1 (theeffective RDSon of the low-side transistor 212)+0.4×0.2 (the effectiveRDSon of the high-side transistor 201)=0.14 ohms, which is smaller thanthe 0.15 ohms calculated above for the 50% duty cycle. Because powerdissipation is proportional to the effective diagonal RDSon,asymmetrical low-side and high-side transistors may reduce power losses.For example, one can see that in at least the above example, theequivalent diagonal RDSon goes down if the duty cycle increases in favorof the transistor with the lower RDSon, in this example, the low-sidetransistor 212. And this may translate into a power savings asillustrated in FIGS. 4 and 5.

FIG. 4 is a plot of power dissipated in drive transistors of FIG. 2illustrating an embodiment of a driving method wherein the drive circuitis approximately symmetrical (all the drive MOSFETs (both high-side andlow-side) are sized approximately the same). This plot assumes a drivingalgorithm that uses a MacDonald waveform over the course of time when adisk is spinning up. Thus, one can see that the low-side MOSFET consumesmore power than the high-side MOSFET because of the nature of theMacDonald waveform (average duty cycle of less than 50%) and due to thefact in this embodiment, the system has a current limiter (not shown)which, to maintain the peak current below a threshold, limits the dutycycle (time when a high-side transistor is ON) to lower number (e.g.,˜20%). That is, the current limiter in this embodiment increases theasymmetry of the PWM ratio. The total power dissipated in the high-sideand low-side transistors as a function of time is approximately equal toa sum of the two plots in FIG. 4.

FIG. 5 is a plot of power dissipated in the high- and low-side drivetransistors of FIG. 2 in an embodiment the drive circuit is asymmetricalin size. This plot also assumes that the transistors are driven with aPWM version of the MacDonald waveform over the course of time when adisk is spinning up. However, the low-side MOSFETs are sizedapproximately 50% larger than the high-side MOSFETs. Thus, one can seethat the low-side MOSFETs still consume more power than the high-sideMOSFETs but less than if they are sized the same as the high-sideMOSFETs per FIG. 4. The power consumed by the high-side MOSFETsincreases in this example, but not enough to overcome the power savingsfrom the reduced power consumption of the low-side MOSFETs. Thus, inthis embodiment, the transistor having the smaller RDSon value is thetransistor that will be ON more of the time than the other transistor.In this embodiment, this decreases the total power dissipation of thedrive circuit 107, and the total power dissipation of the HDD. Theestimated decrease in power dissipation during the spin up is about 3.9%and during the steady state is about 4.7%. The results may be differentif the motor is driven using a waveform other than the MacDonaldwaveform.

In summary, one may decrease the RDSon and effective RDSon value of atransistor by increasing its width-to-length ratio. That is, byincreasing the actual size of the transistor, the RDSon value willdecrease. If die space is not a concern, then one may simply optimizethe power savings of the drive circuit 107 by increasing the size of theMOSFETs that see the longer average ON times. Similarly, one may alsodecrease the sizes of the opposite MOSFETs so long as other driveparameters are still met, e.g., driving current to a respective phase.

In yet another embodiment, one may keep the die size the same bydecreasing the size of the transistors with the higher RDSon value by asmuch as the size increase in the transistors with the lower RDSonvalues. Therefore, one may keep the die size the same yet still realizereduced power consumption. For example, if a symmetrical driver has anequal number of high-side and low-side transistors with normalizedwidths of 1, then the total width of all the transistors is N, where Nis the number of drive transistors. But, an asymmetrical drive circuithaving N/2 transistors of width 1.2 and N/2 transistors of 0.8 also hasa total width of all the transistors equal to N.

Using this technique, one may realize more of a power or die sizesavings the bigger the disk drive, and thus the higher currents andvoltages that are used to drive the motor. That is, one may be morelikely to see a measurable advantage in big server disk drives, and evendesktop computer disk drives, than in smaller disk drives like the diskdrives that go in cameras or other portable devices.

One may also use this technique to reduce transistor size, and thus diesize, without increasing power dissipation. Furthermore, increasing thesize, and thus decreasing the RDSon, of the drive transistors thatconduct most of the load current may increase the yield and reliabilityof the motor controller (or other circuitry that includes the drivetransistors) because at least some failure analysis studies have shownthat the drive transistors conducting most of the load current fail at ahigher rate than the drive transistors conducting less of the loadcurrent.

Another feature of HDD systems is using the disk momentum to generatepower (by using the motor as a generator) to park the head and performother shut down tasks in an emergency or other sudden power-downsituation. Such a technique is described in related U.S. patentapplication Ser. No. 12/505,822 entitled “MANAGEMENT OF DISK DRIVEDURING POWER LOSS” and which is incorporated herein by reference.

In this method, a drive controller 108 (FIG. 1) may turn the high-sideand low-side MOSFETs on and off in a drive algorithm suited to prolongpower generated by the spinning disk when primary power is lost. Assuch, power savings may also be realized in the application of thisdriving algorithm by sizing the particular transistors that are on moreto a have a smaller RDSon value. Therefore, during the disk braking, arelatively large current may flow through the high-side or low-sidedrive transistors. Reducing RDSon in the ones of these transistorscarrying the braking current may then reduce power and heat dissipatedin these transistors.

In one embodiment, there is a decrease in total power dissipation in thedrive transistors, and in the motor controller circuit as a whole,during braking of approximately 16.7%. This allows the circuit to runcooler during braking. Such power savings during braking may provideonly a small advantage to a disk drive during normal operation within adevice (e.g., computer) in which the disk drive is installed, becauseemergency and other sudden shut downs occur relatively infrequently, andbecause the energy stored in the disk momentum will be dissipatedanyway, whether by the braking or by dissipation in the transistors. Butsuch lower power dissipation during braking may provide a largeradvantage during the manufactures' testing of the disk drives, andparticularly of the disk drive heads.

A manufacturer may test the robustness of a read write head during abraking procedure to determine how resilient the head is to bumping intothe rough surface of the disk during the parking procedure. Amanufacturer may do this by cycling the head through a number, e.g.,10,000, emergency/sudden braking cycles. But because the disk drivecontroller (or other circuit used to test the head) has a maximumtemperature rating, the rate at which the head is cycled is slow enoughto prevent the controller temperature from exceeding this maximumrating. Therefore, by reducing the power dissipation through the drivetransistors as discussed above, the manufacturer may increase the cyclerate without exceeding the controller temperature rating, and thus maydecrease the total test time.

FIG. 6 is a block diagram of an embodiment of computer system having theHDD system of FIG. 1. In this embodiment, the HDD system 100 asdescribed above may be part of a computer system 600 having a processor601 coupled to a system memory 602. Such a computer system 600 may be apersonal computer system, server computer system, portable computingdevice, mobile phone, personal data assistant, and the like.

While the subject matter discussed herein is susceptible to variousmodifications and alternative constructions, certain illustratedembodiments thereof are shown in the drawings and have been describedabove in detail. It should be understood, however, that there is nointention to limit the claims to the specific forms disclosed, but onthe contrary, the intention is to cover all modifications, alternativeconstructions, and equivalents falling within the spirit and scope ofthe claims.

What is claimed is:
 1. A controller, comprising: a disk-drive controlleroperable to control operation of a disk drive; and a drive circuitcoupled to the disk-drive controller and operable under control of thedisk-drive controller to control operation of a motor in the disk drive,the drive circuit comprising a first drive component having a firstvalue of a drive characteristic and a second drive component having asecond value of the drive characteristic, the second value beingdifferent than the first value, and the first drive component comprisinga first transistor and the second drive component comprising a secondtransistor and the first and second drive characteristics correspondingto respective RDSon values for the first and second transistors; andwherein the disk-drive controller is operable to asymmetrically activatethe first and second drive components and the first and second values ofthe first and second drive characteristics have values that are afunction of characteristics of the asymmetrical activation to therebyreduce a power loss in the first and second drive components relative toa power loss in the first and second drive components when the first andsecond drive characteristics have the same value, the disk-drivecontroller operable to pulse width modulate the first and secondtransistors in an asymmetric manner to thereby activate the one thefirst and second transistors having the lower RDSon value for a longerduration than the other one of the first and second transistors during acycle of the pulse width modulation.
 2. The controller of claim 1wherein: the first transistor of the first drive component comprises oneof a high side and a low side driver; and the second transistor of thesecond drive component comprises the other of the high side and low sidedriver.
 3. The controller of claim 1, further comprising a firstintegrated circuit die comprising the disk-drive controller coupled to asecond integrated circuit die comprising the drive circuit.
 4. Thecontroller of claim 1, further comprising a single integrated circuitdie.
 5. The controller of claim 2, wherein the first and secondtransistors comprise MOSFET transistors and width-to-length ratios ofthese MOSFET transistors is such that the first MOSFET transistorcomprises a size that is 50% larger than the second MOSFET transistor.6. A disk drive system, comprising: a motor; a disk coupled to the motorand operable to be rotated by the motor; and a motor controller having adrive circuit operable to drive the motor, the drive circuit including:a first drive circuit including a first transistor and having a firstvalue of a drive characteristic corresponding to the RDSon of the firsttransistor; a second drive circuit including a second transistor andhaving a second value of the drive characteristic corresponding to theRDSon of the second transistor, the second value being different thanthe first value; wherein the motor controller is operable to turn on thefirst drive circuit for a first portion of a drive period and to turn onthe second drive circuit for a second portion of the drive period thatis greater than the first portion, the motor controller turning on theone of the first and second transistors having the lower RDSon value forthe second portion and the other one of the first and second transistorsfor the first portion.
 7. The disk drive system of claim 6 wherein themotor comprises a three-phase, brushless direct-current motor.
 8. Thedisk drive system of claim 6, further comprising: a memory coupled tothe drive circuit and operable to store a drive algorithm; and a drivercontroller coupled to the memory and operable to control the drivecircuit according to the stored drive algorithm.
 9. The disk drivesystem of claim 6 wherein the first and second drive circuits compriseMOSFET transistors and width-to-length ratios of these MOSFETtransistors is such that the first MOSFET transistor comprises a sizethat is 50% larger than the second MOSFET transistor.